1. Field Of The Invention
The present invention relates to a viscous isolator, and more particularly to a vibration isolator and damper for isolating a reaction wheel assembly on a supporting structure while permitting a mounting base to be moved relative to the supporting structure along multiple axes of freedom.
2. Description Of The Prior Art
Spacecraft reaction wheel assemblies used for pointing control systems, as on a telescope, can emit vibrations into the spacecraft structure that result in blurred images in optical sensors carried by the spacecraft. A reaction wheel assembly produces vibration disturbances when it rotates due to imperfections in the electromagnetics and their drive electronics, unbalance of the rotor, and imperfections in the spin bearings. Since the reaction wheel assemblies provide a desired control torque for positioning the optical elements in the absence of a chemical reaction propulsion system, the isolation system provide attenuation above specific natural frequencies and must have peaking factors in one or more degrees of freedom without impairing transmission of the desired control torques. To maintain accuracy, vibrations must be isolated between the supported structure and the supporting system during both ground loading (one G) and in orbit (zero G).
One system for isolating vibrations is disclosed in U.S. Pat. No. 3,540,688, wherein a supporting system employs pivoted single-axis isolators arranged into a bipod and a tetrahedron in which the isolators are supported by means of universally rotatable joints to provide a kinematic mount system.
U.S. Pat. No. 4,848,525 discloses an active 6 degree-of-freedom pointing and isolation system having a magnetically suspended positioning system mounted above a hexapod of linear actuators.
Critical to the above isolation systems is the design of the isolator element. Typically, the prior art isolator elements have been designed for relatively high frequencies, of the order of 15 Hz or greater; therefore, deflections due to one G ground loading and launch vibrations were relatively small and not a limiting factor. However, in some applications the vibration isolator is required to provide specific natural frequencies and peaking factors in one or more degrees of freedom. Thus, for an application with a magnetically suspended wheel the radial translation motion was required to be isolated at a relatively low frequency (4.0 Hz), and very tightly controlled frequencies and peaking factors were specified for all six degrees of freedom to avoid interaction with the magnetic suspension control system. Moreover, the isolator was required to operate in both ground and orbit environments. Where the prior art applications utilized a ridgedly attached isolator the radial translational frequency at 4 Hz could not be accurately controlled.
Moreover, viscous damped isolators are designed to produce damping in their axial direction by fluid shear through a controlled annulus. Damping in the radial direction is typically very low as compared to axial damping and not accurately controlled. Therefore, the peaking factor of the radial translational isolator frequency could not be accurately controlled. Further, radial deflection was limited to the radial clearance of the annulus which must be relatively small to achieve good axial and radial damping. A low frequency radial translational isolator requires a larger value of radial motion than can be provided by the prior art isolators. Still further, the small radial gap coupled with the low radial stiffness precludes operation in the radial direction at one G, and results in the isolator chattering between its stops during launch vibration with the resultant transmittal of impact loads to the reaction wheel assembly.
The present invention avoids the foregoing limitations by providing a hexapod of isolation elements pivoted at each end to carry only axial loads. Travel limiting stops are build into the structure so as to avoid transmitting the loads caused by impact during launch to the isolator elements. A tunable viscous damper is provided which may be peaked at a desired resonant frequency.